lib/repo: Change resolve_keyring_for_collection() to return a remote

Instead of returning just the keyring filename, return the entire
OstreeRemote, which has the keyring filename as one of its members. This
will simplify some upcoming changes, and allows slightly improved debug
logging.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

Closes: #1202
Approved by: cgwalters

src/libostree/ostree-repo-pull.c

@@ -5498,7 +5498,7 @@ check_remote_matches_collection_id (OstreeRepo  *repo,
  * Find the GPG keyring for the given @collection_id, using the local
  * configuration from the given #OstreeRepo. This will search the configured
  * remotes for ones whose `collection-id` key matches @collection_id, and will
- * return the GPG keyring from the first matching remote.
+ * return the first matching remote.
  *
  * If multiple remotes match and have different keyrings, a debug message will
  * be emitted, and the first result will be returned. It is expected that the
@@ -5506,10 +5506,11 @@ check_remote_matches_collection_id (OstreeRepo  *repo,
  *
  * If no match can be found, a %G_IO_ERROR_NOT_FOUND error will be returned.
  *
- * Returns: (transfer full): filename of the GPG keyring for @collection_id
+ * Returns: (transfer full): #OstreeRemote containing the GPG keyring for
+ *    @collection_id
  * Since: 2017.8
  */
-gchar *
+OstreeRemote *
 ostree_repo_resolve_keyring_for_collection (OstreeRepo    *self,
                                             const gchar   *collection_id,
                                             GCancellable  *cancellable,
@@ -5517,7 +5518,7 @@ ostree_repo_resolve_keyring_for_collection (OstreeRepo    *self,
 {
   gsize i;
   g_auto(GStrv) remotes = NULL;
-  const OstreeRemote *keyring_remote = NULL;
+  OstreeRemote *keyring_remote = NULL;
 
   g_return_val_if_fail (OSTREE_IS_REPO (self), NULL);
   g_return_val_if_fail (ostree_validate_collection_id (collection_id, NULL), NULL);
@@ -5567,7 +5568,7 @@ ostree_repo_resolve_keyring_for_collection (OstreeRepo    *self,
     }
 
   if (keyring_remote != NULL)
-    return g_strdup (keyring_remote->keyring);
+    return ostree_remote_ref (keyring_remote);
   else
     {
       g_set_error (error, G_IO_ERROR, G_IO_ERROR_NOT_FOUND,